Colin Sullivan was the first to invent the use of nasal Continuous Positive Airway Pressure (CPAP) to treat Obstructive Sleep Apnea (OSA), e.g., see U.S. Pat. No. 4,944,310. The treatment generally provides a supply of air or breathable gas from a blower to a patient via an air delivery conduit and a patient interface, such as a full-face or nasal mask or nasal prongs. The air or breathable gas is commonly delivered at a pressure of 4 cmH2O to 20 cmH2O and acts as a splint to hold the airway open during sleep.
Humidifiers are commonly used with CPAP flow generators to prevent a patient's airways from drying out. The humidified air is generally heated allowing the air to carry more moisture and to provide additional comfort for the patient. A humidifier normally comprises a water tub, a heating element, an air inlet able to receive air from the CPAP flow generator, and an outlet adapted to connect to a delivery conduit to deliver the humidified air through the patient interface to the patient. A humidifier may be integrated or adapted for coupling with a CPAP flow generator. The humidifier is often detachable to allow the water tub to be removed for filling with water.
Humidifier tubs may be washable or disposable. Disposable humidifier tubs are well known in the prior art, e.g., see U.S. Pat. No. 4,203,027 (O'Hare et al.) published 13 May 1980 and U.S. Pat. No. 6,398,197 (Dickinson et al.) published 4 Jun. 2002.
As humidifier systems and tubs become smaller, the surface area of water exposed to the delivered air also inherently becomes smaller, creating problems in maintaining a sufficient moisture pickup by the air flow passing through the tub. The use of baffles between the air inlet and air outlet has been used to encourage the incoming air to flow around the surface of the container and down to the water surface prior to exiting from the air outlet. This suffers from the disadvantage that the air flow suffers a significant pressure drop, thus requiring the flow generator motor to run faster to overcome this drop in pressure and consequently producing more noise.
Another problem with current humidifier tubs is that if the tub is filled above the indicated level, in particular while the tub is connected to the flow generator, the water may flow back into the flow generator.
A further problem with prior art humidifier tubs, especially round tubs, was that users, particularly those users with limited dexterity or poor eyesight or under low light conditions, may have difficulty in correctly orienting the tub for attachment onto the cradle.